Crystal structure and thermal characterization of cadmium oxalate [CdC2O4 · 3H2O] and barium-doped cadmium oxalate [Ba0.5Cd0.5(C2O4)2 · 5H2O] single crystal

Article abstract of DOI:10.1016/j.ica.2008.07.025

Pure cadmium oxalate trihydrate (COT) and barium added cadmium oxalate (BCO) single crystals were grown by controlled diffusion of Cd²⁺ and Ba²⁺ ions in silica gel at ambient temperature. A single test tube technique coupled with gel

Full text of DOI:10.1016/j.ica.2008.07.025

Inorganica Chimica Acta 362 (2009) 1535–1540
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Crystal structure and thermal characterization of cadmium oxalate
[CdC2O4 ꢀ 3H2O] and barium-doped cadmium oxalate
[Ba0.5Cd0.5(C2O4)2 ꢀ 5H2O] single crystals grown in silica gel
b
b
c
d
A. Moses Ezhil Raj a, , D. Deva Jayanthi , V. Bena Jothy , M. Jayachandran , C. Sanjeeviraja
*
a Department of Physics, Scott Christian College (Autonomous), Nagercoil 629 003, India
b Department of Physics, Women’s Christian College, Nagercoil 629 001, India
c ECMS Division, Central Electrochemical Research Institute, Karaikudi 630 006, India
d Department of Physics, Alagappa University, Karaikudi 630 003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pure cadmium oxalate trihydrate (COT) and barium added cadmium oxalate (BCO) single crystals were
grown by controlled diffusion of Cd2+ and Ba2+ ions in silica gel at ambient temperature. A single test tube
technique coupled with gel aging conferred maximum size crystals by controlling the nucleation rate. It
was found that the pH and age of the gel greatly influenced the crystal quality, their size and transpar-
ency. Grown crystals CdC2O4 ꢀ 3H2O and Ba0.5Cd0.5(C2O4)2 ꢀ 5H2O were characterized by X-ray diffraction,
Fourier transform infrared spectroscopy and thermal analysis. Effect of barium dopant on the growth and
morphology of cadmium oxalate was studied. Pure cadmium oxalate crystallized in triclinic system and
the barium-doped cadmium oxalate crystallized in hexagonal system with massive changes in their unit
cell parameters. The infrared spectrum revealed the presence of oxalate ligands and water of hydration in
both the pure and barium-doped crystals. Thermal analysis showed that the grown crystals were dehy-
drated thermally even from lower temperatures and the doped crystals were found more stable.
Ó 2008 Elsevier B.V. All rights reserved.
Received 20 February 2008
Received in revised form 16 July 2008
Accepted 24 July 2008
Available online 30 July 2008
PACS:
81.10.ꢁh
61.66.Fn
33.20.Ea
81.70.Pg
Keywords:
A. Structural materials
D. X-ray diffraction
C. Infrared spectroscopy
D. Thermogravimetry analysis
1. Introduction
Cd and Ba ions have attracted a great deal of interest because it
has rather simple chemistry and coordination geometry [8–13].
Oxalate crystals have attracted the attention of many research-
ers due to their interesting physical properties and their suitability
in preparing ceramic superconductors and solid solutions [1–4].
The formation of solid solutions of some multi-metal oxalates have
been discussed in detail by Schuele [5] and Fischer et al. [6]. More-
The effect of doping on gel-grown crystals has been extensively
studied by Dishovsky and Boncheva-M Ladenova [14] and Dennis
and Henisch [15]. Mixed crystals are reported to be harder than
pure ones [16,17]. In addition to growth parameters, additives such
as PSMA (poly-styrene-alt-maleic acid) [18], PSSS (poly sodium 4-
styrene-sulfonate) [19] and sodium carboxylate [20] on oxalate
crystal matrix have been studied to identify the phase and struc-
tural modifications. In an ongoing investigation of the crystal
chemistry of oxalates [21–24], the present study is aimed to crys-
tallize cadmium oxalate trihydrate (COT) and barium-doped cad-
mium oxalate (BCO) mixed crystals in gel media in order to
study the effect of gel parameters on nucleation, structural modifi-
cations and thermal decomposition behavior.
2ꢁ
over, oxalate ions C2O4 can act as monodentate, bidentate, tri-
dentate or tetradentate donor ligands and therefore, it was found
to show a large variety on structural characterizations. In addition,
they can form chains, layers and 3-D networks on combining with
the metal centers. The water of hydration that links the metal ions
and/or loosely bonded to the framework leads to large variety of
structural architecture in multi-dimensions [7]. Consequently, con-
certed attention has been devoted to grow divalent metal oxalates
using various techniques. Among the divalent metal ions studied,
2. Experimental
In gel technique, silica gel has been used to grow organic and
inorganic crystals because of their porous network that permits
* Corresponding author. Tel.: +91 04652 232888; fax: +91 04652 229800.
E-mail address: ezhilmoses@yahoo.co.in (A.M. Ezhil Raj).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.07.025

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A.M. Ezhil Raj et al. / Inorganica Chimica Acta 362 (2009) 1535–1540
Table 1
the diffusion of different ions of any size. Basically silica gel of spe-
cific gravity (1.04) was obtained by the neutralization of sodium
metasilicate solution with oxalic acid. Gels of different pH (3.5,
4.0 and 4.5) were prepared and the reproducibility was tested by
noting the gelation time of gels. After a gel aging of three weeks,
growth of cadmium oxalate trihydrate was achieved by allowing
controlled diffusion of cadmium ions (Cd2+) through silica gel
impregnated with oxalic acid without damaging the gel surface.
The tube was kept undisturbed at room temperature and the
chemical reaction leads the formation of cadmium oxalate.
Optimized conditions for the growth of pure and barium-doped cadmium oxalate
crystals
Parameters
Cadmium
oxalate
Barium-doped cadmium
oxalate
Concentration of oxalic acid
Concentration of cadmium
chloride
Concentration of barium
chloride
Specific gravity of gel
Gel pH
Gel setting period
Gel aging
1 M
1.5 M
1 M
1.5 M
1.5 M
1.04
3.5–4.5
10 h
3 weeks
30 days
RT
1.04
3.5–4.5
10 h
3 weeks
30 days
RT
CdCl2 þ H2C2O4 ! CdC2O4 þ 2HCl
ð1Þ
Barium added cadmium oxalate crystals were grown by mixing
1.5 M CdCl2 and 1.5 M BaCl2 in equal proportion and placed as the
supernatant solution and is allowed to diffuse into the gel medium
containing oxalic acid. The following chemical reaction was ex-
pected in the growth of barium-doped cadmium oxalate (BCO)
crystals.
Period of growth
Temperature
Growth rate variations and nucleation control using gel-aging
technique were observed. Crystal structure of both COT and BCO
were analyzed using an X’pert PRO X-ray diffractometer
0:5BaCl2 þ 0:5CdCl2 þ 2H2C2O4 ! Ba0:5Cd0:5ðC2O4Þ2 þ 4HCl
ð2Þ
(
D
2h = 10–70°, 0.2° as increment, integration time 1 s and Cu
Cadmium oxalate crystals were transparent with many facets,
whereas on addition of Ba2+, spherulite coalesced transparent crys-
tals were harvested (Fig. 1). The maximum sizes of the crystals
grown were about 0.44 ꢂ 0.42 ꢂ 0.31 cm in the case of pure cad-
mium oxalate crystals and 0.33 ꢂ 0.21 ꢂ 0.20 cm in the case of
doped oxalate crystals. The optimized growth conditions for the
growth of pure and barium-doped cadmium oxalate crystals are
summarized in Table 1.
K
a1 radiation, k = 1.5406 Å). FT-IR spectra of the oxalate crystals
in the wave number range of 400–4000 cmꢁ1 were recorded on a
Bruker Vector 22 spectrometer using KBr pellet technique. The
thermal decomposition behavior of the individually precipitated
COT and co-precipitated BCO crystals were studied by means of a
TA instrument, Model Perkin Elmer, TGA 7 (US) (temperature
range: ambient to 700 °C) at the heating rate of 5 °C/min.
Starting with gelation time and their variation with pH, the
growth pattern was observed for undoped and doped crystals.
3. Results and discussion
3.1. Crystal growth observations and nucleation control
a
b
During the growth of crystals, initial crystallization occurred
just below the gel–solution interface. In the beginning, very small
crystals without definite shape were found to grow and the crystal
nucleation density was high due to higher supersaturation at the
interface. In due course, as metal ions diffused deep into the gel
and due to the nonavailability of sufficient cations, the number
of crystals decreased and large crystals with definite morphology
resulted far below the interface. Growth of the crystal was stagnant
after 30 days. In order to investigate the exact growth conditions,
the experiment was repeated by changing the growth parameters.
3.1.1. Gelation time as a function of pH
The gelation time depends on many parameters such as concen-
tration of ionic species inside the gel, gel pH, and temperature of
the gel. In our study, gels were prepared for the pH values 3.5,
4.0 and 4.5, whereas other parameters like concentration of ionic
species and crystallization temperature were held constant. On
observing the gelation time variation with pH for silica gels, the
low pH gels (acidic gels) took more time to set than the high pH
gels. Usually polymerization reaction is very fast at the neutraliza-
tion point and therefore gelation time is very low for the high pH
gels. Low pH gels are far from neutralization and therefore the
gelation time is high. The observed gelation time was 26.5 h. for
the 4.5 pH gel, whereas 40 h for the gel of pH 3.5.
3.1.2. Growth observations
Fig. 1 shows the growth pattern of pure and barium-doped oxa-
late crystals grown for identical crystallization parameters: gel age,
3 weeks; supernatant solution concentration, 1.5 M; and reactant
solution concentration in gel, 1 M. The minimum value of pH for
which crystals of cadmium oxalate crystallized was 3.5. In all the
experiments, the upper reactant on contact with the gel pre-
charged with lower reactant was observed to form a white precip-
Fig. 1. (a) Liesegang ring formation in the growth of cadmium oxalate and (b)
spherulite crystals of barium-doped cadmium oxalate.

A.M. Ezhil Raj et al. / Inorganica Chimica Acta 362 (2009) 1535–1540
1537
itate which advanced into almost the entire gel column leaving a
small transparent zone at the bottom. The precipitate did not form
a continuous band. Instead, the precipitated column was divided
into several bands, each thicker band of dense precipitate sepa-
rated by a partially transparent band of relatively thinner precipi-
tate. As a result, faintly observable rings against the background of
the precipitate were obtained. As days advanced, clear transparent
zone results because the colloidal particles of the species get con-
densed into crystals. Finally, Liesegang ring formation is observed
and below that, the density of nucleation was highly controlled
and isolated nucleation sites were observed as seen in Fig. 1a. As
metal ions diffused deep into the gel, the number of crystals de-
creased and large crystals with definite morphology resulted.
Within the gel pH studied, crystallization of cadmium oxalate fa-
vors the Liesegang ring formation, whereas on barium doping ring
formation is not observed (Fig. 1b).
acteristic of a one-dimensional diffusion control process. However
at the early stages of growth, a departure in linearity is observed.
This may be due to transient period during which the steady state
concentration are established. The linear parabolic middle region
reveals the supersaturation of the species needed for the growth
process. The self sufficient supply of the required species results
in the formation of highly perfect single crystals that confirms
the constant surface supersaturation hypothesis [25]. During the
final growth stage, the curve deviates from its linearity due to
the shortage of the available solute for the formation of the re-
quired compound.
3.1.4. Nucleation rate as a function of gel pH
Nucleation rate was estimated during the growth of the crys-
tals. It is clear from Fig. 3 that the pH of gel drastically changes
the nucleation rate of the crystals. This variation is attributed to
the variation in pore size of the gel matrix. Because of the varia-
tions in pore size, the gel structure controls the convective mass
flow while allowing the Brownian motion of ions and small clus-
ters through the intra-porous phase. In both the nucleation of cad-
mium oxalate and barium-doped cadmium oxalate crystals, gels
with more acidic pH show less number of nucleation sites. Conse-
quently, it is possible to say that gels with acidic pH have a more
homogeneous porous size distribution with small pore radius
[26]. Further in homogeneous nucleation, molecules come together
and form a stable nucleus that grows in size to form critical nucle-
ation that obeys the homogeneous nucleation theory. It is envis-
aged that after the formation of critical nucleation, continuous
growth is possible only if sufficient solute is available. Supersatura-
tion (S) is required for the formation of crystals and it should obey
the energy consideration as given by Gibbs [27,28].
It is evident that as the droplet density increases, the supersat-
uration decreases, which will lead to a decrease in nucleation den-
sity. This case favors the controlled growth of crystals in low pH
gels. Moreover, the nucleation rate increases rapidly during the
early stages of observation and then it ceases at the end stages
and move towards saturation. Comparatively the nucleation rate
is approximately five times more in barium-doped cadmium oxa-
late than pure cadmium oxalate crystals.
3.1.3. Growth rate as a function of time
Fig. 2 shows the crystal size variations of the grown pure and
barium-doped crystals in gels of pH 3.5. Growth rate curves of both
COT and BCO represent a portion of a parabola, which is the char-
Cadmium oxalate
Barium added cadmium oxalate
0.4
0.3
0.2
0.1
0
50
100
150
200
250
3.1.5. Nucleation control using gel aging
Time (hrs.)
Control of nucleation is of great importance because crystals
that grow in any particular gel system compete with one another
for solute. The competition reduces their size, perfection and
Fig. 2. Crystal size variations as a function of time for COT and BCO.
pH 3.5 (COT)
pH 4.0
200
160
120
80
pH 3.5 COT
pH 4.0
BCO
400
pH 4.5
pH 3.5 (BCO)
pH 4.0
pH 4.5
pH 4.5
BCO
pH 3.5 BCO
pH 4.0
pH 4.5
300
200
100
COT
COT
40
0
1.0
1.5
2.0
2.5
3.0
0
50
100
150
200
250
Growth duration (hrs.)
Gel aging (weeks)
Fig. 3. Number of nuclei (N) vs. growth duration.
Fig. 4. Variation of nucleation on gels of different age.

1538
A.M. Ezhil Raj et al. / Inorganica Chimica Acta 362 (2009) 1535–1540
purity. By adopting gel-aging technique, the nucleation sites can be
controlled by placing the overhead solution after a predefined per-
iod above the set gel. A typical variation of nucleation with gel
aging is shown in Fig. 4 for both the COT and BCO crystals. As seen,
the nucleation rate decreases as the age of the gel is increased. This
may be due to syneresis, during which the pore size gets reduced
further so that less amount of reactant is transported through
the gel for the formation of crystals that leads to controlled growth
with less number of nucleation sites. In the present study, purity of
the crystal is retained only by adopting the gel-aging technique
and by optimizing the pH of the gel.
crystal is hexagonal in structure and the observed lattice parame-
ters are in good agreement with the reported values [28]. The crys-
tal data of both COT and BCO crystals are given in Table 2. The
microstrain (r) [31] and dislocation density (d) [32] of COT crystal
in its preferred orientation (010) is found to be 2.9 ꢂ 10ꢁ3 and
9.67 ꢂ 1014 lines/m2, respectively. Whereas, 2.6 ꢂ 10ꢁ3 and
4.35 ꢂ 1014 lines/m2 are the respective values of ‘
r’ and ‘d’ of bar-
ium-doped cadmium oxalate crystals in its (101) lattice plane. It is
interesting to note that the crystal structure of pure COT changes
from triclinic to hexagonal when barium is added in 1:1 (Cd:Ba) ra-
tio in the supernatant precursor solution.
3.2. Structural characterization
3.3. Infrared spectral studies
The XRPD (powder XRD) pattern of COT and BCO crystal are
shown in Fig. 5. The pattern (Fig. 5a) shows that the sample is
essentially in single phase and the lines are indexed on the basis
of triclinic structure of pure COT according to the Joint Committee
on Powder Diffraction Standards (JCPDS Card No. 53-0085) and the
reported results [29,30]. However, X-ray diffractogram obtained
for BCO presents coincident dhkl values with Ba0.5Cd0.5(C2O4)2 ꢀ
5H2O (JCPDS Card No. 45-0743). The XRD data prove that the
FT-IR vibrational spectra of CdC2O4 ꢀ 3H2O (Fig. 6a) and
Ba0.5Cd0.5(C2O4)2 ꢀ 5H2O (Fig. 6b) crystals grown in silica gel exhib-
its strong absorption peaks in between 3400 and 3500 cmꢁ1, which
is due to OH stretching vibrations of water. On adding barium as
dopant, the shift in the band from its usual position to the lower
wave number side is observed, which may be due to the OH bond
weakness caused by the interaction of the additionally added me-
tal ion Ba2+ in the crystal lattice. The bands at approximately
Fig. 6. FT-IR spectra of (a) cadmium oxalate and (b) barium added cadmium
oxalate.
Fig. 5. XRPD pattern of (a) COT and (b) barium-doped COT crystal.
Table 2
Obtained crystallographic parameters for cadmium oxalate single and compound patterns
Compound
Crystal system
Spatial group
Unit cell parameters
Observed
JCPDS standards
53-0085
Standard
ꢀ
CdC2O4 ꢀ 3H2O
Triclinic
P1(2)
a = 5.9903(7) Å
b = 6.6484(2) Å
c = 8.5728(4) Å
a = 6.0059 (4)Å
b = 6.6656(3) Å
c = 8.4925(6) Å
a
= 73.984(8)°
b = 74.196(4)°
= 80.840(2)°
a
= 74.660(5)°
b = 74.287(6)°
= 81.008(6)°
c
c
BaCd2(C2O4) ꢀ 5H2O
Hexagonal
a = 8.358 Å
c = 13.19 Å
a = 8.37 Å
c = 13.42 Å
45-0743

A.M. Ezhil Raj et al. / Inorganica Chimica Acta 362 (2009) 1535–1540
1539
1600 cmꢁ1 are attributed to the C@O stretch of the carbonyl group
and the peaks at around 1300 cmꢁ1 is assigned to C@O symmetric
and O–C@O modes [33–37]. Even though C and H molecules are
present in the oxalate matrix, the absence of bands in the 1145,
1050, 1006, 816 cmꢁ1 reveals the purity of the grown oxalate crys-
tals without any C–H bonding. The bands below 800 cmꢁ1 are due
to metal–oxygen bonds. As seen, barium-doped COT has more
number of absorption bands in the lower wave number region
(<800 cmꢁ1), which reveals the incorporation of barium ions in
the crystal lattice forming additional metal–oxygen (Ba–O) bond-
ing. Table 3 summarizes the FT-IR results of the pure and barium
added cadmium oxalate crystals.
3.4. Thermal analysis
In order to elucidate the decomposition behavior of the pure
and coprecipitated oxalate crystals, TG/DTG curves of COT and
BCO crystals are given together for comparison (Fig. 7). The shape
of thermal decomposition curve of CdC2O4 ꢀ 3H2O in the tempera-
ture range 31–179 °C shows a single step dehydration reaction.
Table 3
Vibrational modes, in wavelength (cmꢁ1), observed in the infrared spectra for the
samples Cd(C2O4) ꢀ 3H2O and BaCd(C2O4)2 ꢀ 5H2O
Vibration modes (cmꢁ1) observed in
Fig. 7
Literature
data
Assignment of peaks/
bands
Cd(C2O4) ꢀ 3H2O BaCd2(C2O4) ꢀ 5H2O
Fig. 8. DTA of (a) cadmium oxalate and (b) barium added cadmium oxalate crystals.
3542
3496
1613
1381
1314
777
3482
3429
1586
1348
1301
791
3502 [28]
1620 [29]
1393 [30]
1314 [28]
804 [29]
c
HOH
dH2O
c
c
(CO) + (CC)
(CO) + d(O–C@O)
The measured weight loss for this decomposition stage is about
20.79% of the total weight (calculated loss: 21.24%) resulting in
the elimination of three water molecules. Corresponding to this
dehydration step, there is only one endotherm in DTA (Fig. 8a).
In the second stage (179–390 °C), dehydrated cadmium oxalate is
reduced to carbonate and DTA confirms the carbonate formation
followed by the exothermic disproportionation [38]. In the third
and the final stage (not shown in figure), the material is reduced
to cadmium oxide (>700 °C).
545 [28]
d(O–C@O) +
c(MO)
c(MO)
519
532
479
Co-precipitated Ba0.5Cd0.5(C2O4)2 ꢀ 5H2O has the first stage of
dehydration (32–333 °C), which consist of three sub-stages in the
temperature ranges 32-118 °C, 118–197 °C and 197–333 °C
(Fig. 8b). These dehydration stages are occurred at higher temper-
atures than the dehydration of pure CdC2O4 ꢀ 3H2O which reveals
the stability criterion of the doped crystals [16,17]. The first sub-
stage (32–118 °C) is due to loss of one H2O molecule and the sec-
ond sub-stage (118–197 °C) results in the elimination of two water
molecules from the material. In the third and final sub-stage (197–
333 °C), remaining two water molecules are ejected out. The total
measured weight loss in the stage (32–333 °C) is about 14.33%,
resulting in the elimination of five water molecules. Corresponding
to these dehydration steps, three endothermic peaks are observed
in DTA (Fig. 8b), at 78, 175 and 221 °C, respectively. In the second
stage, the mass loss between 333 and 425 °C corresponds to the
formation of carbonates from anhydrous mixed oxalate by the evo-
lution of CO. As evidenced from DTA, this decomposition stage also
has two sub-stages. The exotherm at 386 °C can be attributed to
the partial decomposition of the Ba + Cd oxalate into barium car-
bonate followed by the decomposition of the remaining mixed
oxalate into a homogeneous mixed Ba + Cd carbonate at 452 °C.
Knaepen et al. [39] and Dharmaprakash and Mohan Rao [40,41]
have already reported similar observation for the decomposition
of Ca + Sr oxalate crystals. These results revealed a fact that partial
segregation takes place during the decomposition process. On
heating further above 700 °C, partially segregated mixed carbonate
Fig. 7. TGA of (a) Cadmium oxalate and (b) barium added cadmium oxalate crystals.

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4. Conclusions
Pure and barium added cadmium oxalate single crystals were
grown by the gel method. The optimized crystallization conditions
were: gel pH, 3.5; specific gravity of the gel, 1.04, gel age, 3 weeks;
lower reactant concentration, 1 M; upper reactant concentration,
1.5 M; 1.5 M CdCl2 + 1.5 M BaCl2, mixed in 1:1 ratio. The growth ki-
netic studies revealed the dependence of nucleation density, mor-
phology and ultimate size of crystals on growth parameters like gel
concentration, gel age, gel pH, and concentration of lower and
upper reactants. The decrease in supersaturation resulted less
nucleation in aged gels. X-ray power diffraction has revealed the
crystallinity of the material. On adding barium, the triclinic struc-
ture of the COT transformed to hexagonal structure. Presence of
water of crystallization and the oxalate phase formation was iden-
tified from the FT-IR band assignments. The thermogravimetric
analysis hinted that the gel-grown cadmium oxalate had three
molecules of water of hydration and the barium added crystals
had five molecules of water of hydration. The chemical formulae
CdC2O4 ꢀ 3H2O and Ba0.5Cd(0.5)(C2O4)2 ꢀ 5H2O were established from
the experimental evidences.
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